Micromechanical pressure sensors for measuring the pressure in a medium have been designed and also methods for manufacturing micromechanical sensors are described in literature. Often the substrate in which those sensors are produced is made of semiconductor material.
Differential or relative pressure sensors measure the pressure difference between two pressure environments. Typically, one of the pressure environments can be the environment's ambient pressure (as opposed to a second, pressurized environment). Absolute pressure sensors measure pressure with respect to a zero pressure reference value (vacuum reference).
A cutting-edge precision pressure-sensing technology of interest is a piezo resistive measurement technique. Piezo resistive materials have the ability to convert mechanical stress into a change of electrical properties For a pressure sensor, a semiconductor membrane is provided, and a p-type diffusion layer of piezo resistive material is formed on top of it to make a resistive layer on the membrane. The pressure on the semiconductor membrane causes the surface stress on the semiconductor material changes the resistance value. The signal changes are then amplified and interpreted as pressure readings.
Relative pressure sensors have an open back side under the membrane and the pressure sensor gives an indication of the pressure difference between the front and the back side of the membrane.
For absolute pressure sensors the back side of the membrane is vacuum sealed. The pressure in the vacuum cavity at one side of the membrane is the reference pressure which allows to make an absolute pressure measurement. Absolute pressure sensors are for example useful for measuring the pressure in a tire.
For certain applications, however, the need exists to measure an absolute and a relative pressure at the same time. Therefore, there is a need for a robust absolute pressure sensor as well as a robust relative pressure sensor present on a same die and for methods of manufacturing such systems.